[supdudesupdude]

Weather soundings play a part in making a forecast but it's not the only thing? Many times per day we collect weather station data, correlate against past forecasts, bias correct, etc.

Forecasts are made of so much input data it's insane, yes balloons matter but it's not the only thing. They are the only decent source of conditions aloft and the jet stream is main controller of our surface weather so it makes sense.